MAP

Saturday, 15 June 2013

"We're accurately representing the real object and
calculating the light an astronomer would actually see," says Scott Noble,
associate research scientist in RIT's Center for Computational Relativity and
Gravitation. "This is a first-of-a-kind calculation where we actually
carry out all the pieces together. We start with the equations we expect the
system to follow, and we solve those full equations on a supercomputer. That
gives us the data with which we can then make the predictions of the X-ray
spectrum."

Lead researcher Jeremy Schnittman, an astrophysicist at NASA's
Goddard Space Flight Center, says the study looks at one of the most extreme
physical environments in the universe: "Our work traces the complex
motions, particle
interactions and
turbulent magnetic fields in billion-degree gas on the threshold of a black
hole."

By analyzing a supercomputer simulation of gas flowing
into a black hole, the team finds they can reproduce a range of important X-ray
features long observed in active black holes.

"We've predicted and come to the same evidence that
the observers have," Noble says. "This is very encouraging because it
says we actually understand what's going on. If we made all the correct steps
and we saw a totally different answer, we'd have to rethink what our model
is."

Gas falling toward a black hole initially orbits around
it and then accumulates into a flattened disk. The gas stored in this disk
gradually spirals inward and becomes compressed and heated as it nears the
center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million
C)—some 2,000 times hotter than the sun's surface—the gas shines brightly in
low-energy, or soft, X-rays.

For more than 40 years, however, observations show that
black holes also produce considerable amounts of "hard" X-rays, light
with energy 10 to hundreds of times greater than soft X-rays. This
higher-energy light implies the presence of correspondingly hotter gas, with
temperatures reaching billions of degrees.

The new study bridges the gap between theory and
observation, demonstrating that both hard and soft X-rays inevitably arise from
gas spiraling toward a black hole.

Working with Noble and Julian Krolik, a professor at
Johns Hopkins, Schnittman developed a process
for modeling the inner region of a black hole's accretion disk, tracking the
emission and movement of X-rays, and comparing the results to observations of
real black holes.

Noble developed a computer simulation solving all of the
equations governing the complex motion of inflowing gas and its associated
magnetic fields near an accreting black hole. The rising temperature, density
and speed of the infalling gas dramatically
amplify magnetic fields threading through the disk, which then exert additional
influence on the gas.

The result is a turbulent froth orbiting the black hole
at speeds approaching the speed of light. The calculations simultaneously
tracked the fluid, electrical and magnetic properties of the gas while also
taking into account Einstein's theory of relativity.

Running on the Ranger supercomputer at the Texas
Advanced Computing Center located at the University of Texas in Austin, Noble's
simulation used 960 of Ranger's nearly 63,000 central processing units and took
27 days to complete.

Over the years, improved X-ray observations provided
mounting evidence that hard X-rays originated in a hot, tenuous corona above
the disk, a structure analogous to the hot corona that surrounds the sun.

"Astronomers also expected that the disk supported
strong magnetic fields and hoped that these fields might bubble up out of it,
creating the corona," Noble says. "But no one knew for sure if this
really happened and, if it did, whether the X-rays produced would match what we
observe."

Using the data generated by Noble's simulation, Schnittman and Krolik developed tools to
track how X-rays were emitted, absorbed and scattered throughout both the
accretion disk and the corona region. Combined, they demonstrate for the first
time a direct connection between magnetic turbulence in the disk, the formation
of a billion-degree corona, and the production of hard X-rays around an
actively "feeding" black hole. Results from the study, "X-ray
Spectra from Magnetohydrodynamic Simulations of
Accreting Black Holes," were published in the June 1 issue of The
Astrophysical Journal (ApJ, 769, 156).

In the corona, electrons and other particles move at
appreciable fractions of the speed of light. When a low-energy X-ray from the
disk travels through this region, it may collide with one of the fast-moving
particles. The impact greatly increases the X-ray's energy through a process
known as inverse Compton scattering.

"Black holes are truly exotic, with extraordinarily
high temperatures, incredibly rapid motions and gravity exhibiting the full
weirdness of general relativity," Krolik says. "But our calculations show we can understand
a lot about them using only standard physics principles."

The study was based
on a non-rotating black hole. The researchers are extending the results to
spinning black holes, where rotation pulls the inner edge of the disk further
inward and conditions become even more extreme. They also plan a detailed
comparison of their results to the wealth of X-ray observations now archived by
NASA and other institutions. Black holes are the densest objects known.
Stellar-mass black holes form when massive stars run out of fuel and collapse,
crushing up to 20 times the sun's mass into compact objects less than 75 miles
(120 kilometers) wide.